FINDING THE FIT. Shortly after announcing the discovery of a new extrasolar planet orbiting Upsilon Andromedae in 1997, the researchers became convinced that there was something missing in the picture. An analysis of 11 years of observations finally provided the answer: there is not one planet but three. Here is how computer modeling helped them pinpoint two more planets from their Doppler spectroscopy data.

SHORT-TERM PLOTS of the first planet orbiting Upsilon Andromedae, designated companion b, look conclusive. In any few months, the spectrographic data show it zipping around the planet every 4.6 Earth-days. But when the investigators looked at longer time periods, the data became "noisy." Suspecting that there might be another object in the system, the principal discoverers--Geoffrey W. Marcy of San Francisco State and R. Paul Butler of the Anglo-Australian Observatory--notified other astronomers.

SECOND TRY. The researchers then plugged 11 years of observations into a computer model to plot the characteristics of the system if it consisted of two planets. The data produced an imperfect fit, with many points falling outside the curve. Was there yet another object?

FINAL RESOLUTION. To determine if the odd data points were the signal of a third planet, the team ran a model for three planets. This time, almost all the points fit within the predicted curves.

By subtracting the data from any two of the curves, the researchers were able to obtain orbital plots for each of the three planets.

Images: San Francisco State University

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